The present invention pertains to an aqueous inkjet ink comprising carbon black pigment stabilized by a certain block copolymer dispersant. The invention further pertains to an ink set comprising this ink and to a method of printing with the ink set.
Inkjet printing is a non-impact printing process in which droplets of ink are deposited on a substrate, such as paper, to form the desired image. The droplets are ejected from a printhead in response to electrical signals generated by a microprocessor.
Most inkjet printers are equipped with an ink set comprising two or more different inks and are able to print black text and multicolor images. Typically, an ink set will comprise at least a cyan, magenta and yellow colored ink and a black ink (CMYK ink set).
For printing text, it is desirable for the black ink to have high optical density. For this purpose, a pigment colorant is most advantageous, especially a carbon black pigment. A pigment colorant is not soluble in the ink vehicle and must be treated in order to cause it to remain stably dispersed and jet properly.
To effect stable dispersion, pigments are commonly treated with dispersants, and a wide variety of such materials have been disclosed. Especially effective are block copolymer dispersants which are described, for example, in U.S. Pat. No. 5,085,698. Other examples include benzylmethacrylate (BZMA)//methacrylic acid (MAA) 13//10 block copolymer with number average molecular weight (Mn) of 2966 disclosed in U.S. Pat. No. 5,852,075 and benzylmethacrylate (BZMA)//methacrylic acid (MAA) 13//3 block copolymer with number average molecular weight (Mn) of 2522 disclosed in U.S. patent publication US2005/0090599.
For CMYK ink sets comprising a pigment black ink, it is known to print the black ink in an overlapping relationship with one or more of the colored inks to improve the print quality of the black ink when the one or more colored ink(s) are formulated with an ingredient that destabilizes the black pigment dispersion. Such ink sets and printing method are disclosed for example in U.S. Pat. Nos. 5,734,403 and 6,354,693, and in European Patent Publication 1,125,994 A1.
Despite the successful inkjet inks and print methods presently available, there is still a need for, and it is an objective of this invention to provide, inks and methods having even better print quality and jetting reliability.